The direct stimulatory effect of vitamin A (retinol) on spermatogenesis suggests a locally mediated intracellular role of retinol within the seminiferous tubules (ST). The proposed study is to test the hypothesis that within the ST the Sertoli cell (Sc) is the primary cell responsive to the retinol-mediated effect on spermatogenesis. To test this hypothesis, the following sequence of studies is planned: 1) Precise localization of retinol within the ST will be determined using autoradiography (Study #1); 2) In Study #2 it will be determined which the Sc cytoplasm contains retinol binding protein (RBP) that has a high affinity and low binding capacity to retinol; 3) Study #3 deals with determination whether the Sc surface membrane contains specific retinol receptors to bind with retinol/RBP complex; 4) Study #4 deals with isolation of the developmental stages (spermatocyte and spermatid) during spermatogenesis and determining whether retinol is present in any or all stage(s). In Study #1 tritiated retinol of high specific activity will be injected into the spermatic artery in adult rats and in rats with Sc-enriched testes. Autoradiographs from entire cross sections of the testes will be examined to determine retinol localization within the ST. In Study #2 Sc-enriched testes will be used to isolate RBP from the Sc cytoplasm using the sucrose density gradient method. Binding of RBP to retinol will be determined by saturation assays. In Study #3 apo-RBP (retinol-free RBP) will be prepared, iodinated, and incubated with Sc isolated from Sc-enriched testes to determine whether the Sc contain surface receptors to bind apo-RBP. In Study #4 labeled retinol will be injected into the spermatic artery to determine whether labeled retinol (or its metabolites) is incorporated into the developing spermatocytes and/or spermatids.